1. Field of the Invention
The present invention relates to a position measurement unit, a measurement system and a lithographic apparatus comprising such position measurement unit.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithographic apparatuses, position measurements, e.g. of a position of the substrate table, in a plural degrees of freedom (DOF) are required. Examples of such position measurement systems are described in US 2004/0263846, which is included herein by reference. This document describes measurement systems to determine a position of the substrate table in up to 6 degrees of freedom, the measurement systems comprising a combination of optical interferometers and encoders. In this document, a plane in which the substrate table moves when performing scanning or stepping movements, is defined as an X, Y plane while a dimension perpendicular thereto has been defined as a Z plane. The X, Y plane substantially corresponds to a surface of a substrate held by the substrate table. Also, US 2004/0263841 describes measurement systems to determine a position of a mask table to hold the mask or reticle, in up to 6 degrees of freedom. Here similarly, a surface of the mask substantially coincides with the X, Y plane, while the dimension is substantially perpendicular thereto. In the examples shown, the measurement system comprises a combination of interferometers and encoders. Generally speaking, in some of the examples given here, the interferometers are applied to determine a position (or in fact more precisely to determine a distance) in the X dimension or Y dimension, while the encoders are generally speaking applied to determine a distance in the Z dimension. A range of movement of the substrate table or the mask table may be, during operation of the lithographic apparatus, far larger in the X and Y dimension than in the Z dimension. Also, accuracy requirements may differ per dimension. In another example, as shown in FIGS. 6 and 7 of this document, encoders are applied for measuring a position of the substrate table in X and Y dimension, while an interferometer is applied for measuring the position of the substrate table in the Z dimension. As shown in FIGS. 6 and 7, for each dimension a separate measurement unit is applied. Commonly, it is desired to measure a position of e.g. the substrate table in 6 degrees of freedom, which would require, making use of the solutions as presented in this document, at least 6 measurement units (comprising either an interferometer or an encoder).
One of the ever increasing demands on a lithographic apparatus is to increase a yield thereof, which translates into a higher member of the wafers to be processed in a certain timespan, hence a faster handling, shorter illumination time, etc. of each substrate to be processed. To achieve such a goal, it is desired to increase a speed of movement (e.g. a scanning speed or stepping speed) of the substrate table to allow a faster irradiation of the surface of the substrate. Also, a diameter of the wafer or substrate tends to increase with every generation of lithographic apparatuses. Now, conflicting requirements come into existence, as high speeds of movement of the substrate table on the one hand require a low mass thereof, while on the other hand increasing diameters of the wavers require a more large substrate table, which translates into a higher mass thereof. These conflicting requirements are even worsened by the measurement solutions are presented in US 2004/0263846, as each of the detectors (interferometers or encoders) requires an additional area on a surface of the substrate table, thus increasing a dimension and a weight of the substrate table even further.
A further aspect which comes forward in the measurement solutions as presented in FIGS. 6 and 7 of US 2004/0263846 is that the individual interferometers, encoders, etc. are to be aligned with respect to each other to remove errors due to e.g. misalignments, etc. between them. Also, the physical distance between the beams of the interferometers and encoders in for example the solution presented in FIG. 7 of the document referred to above, may lead to additional measurement errors in case that the grid is not absolutely parallel to the surface of the substrate table, hence requiring a calibration to correct for these errors.
Examples of a refraction type encoder are described in Digitale Laengen- und Winkelmesstechnik: Positionsmesssysteme fuer den Maschinenbau und die Elektronikindustrie, Alfons Ernst [Heidenhain], (1998), as well as in Laengen in der Ultrapraezisionstechnik messen, Alfons Spies, Feinwerk & Messtechnik 98 (1990) 10 page 406-410, which are included herein by reference.